Understanding fast-acting and slow-acting carbohydrates

Carbohydrates are one of the most important elements of a diet for people with diabetes.¹ But why does lemonade, for example, have such a fast effect on blood glucose levels, but the effect of a slice of whole-grain bread is so slow? Keep reading to learn about how carbohydrates are processed.

Carbohydrates are often the subject of controversy. Some people cannot imagine a meal without carbohydrates, such as bread or pasta. Others eat low-carb meals, meaning they avoid fast-acting carbohydrates in particular and also limit their consumption of slow carbs. But what actually are carbohydrates, what foods contain them, and what functions do they provide for the human body?

Fast-acting and slow-acting carbohydrates: similarities and differences

Along with fats and proteins, carbohydrates are one of the three principal nutrients in foods.² All of them can supply the body with energy, with carbohydrates delivering 4 kilocalories (kcal) per gram.¹

Both slow-acting and fast-acting carbohydrates consist of one or more (compound) sugar molecules, which means they have an effect on blood glucose levels as well as the insulin secretion associated with them.²

Carbohydrates are found predominantly in plant-based products.² They mostly occur naturally in grain, potatoes, legumes, vegetables, fruit, and products made from them. However, dairy products, sweets, honey, sweet spreads, drinks such as lemonade, and juices also contain mostly rapidly processed carbohydrates.³

Did you know? Slow-acting, long-chain carbohydrates, such as those found in whole-grain products, cause blood glucose levels to rise gradually.⁴ Fast-acting, short-chain carbohydrates, such as those found in juices or sweets, cause fast increases.

Processing and function of fast-acting and slow-acting carbohydrates

Carbohydrates, particularly slow-acting ones derived from natural products, primarily supply energy and should make up around 40-70 percent of your daily calorie intake.⁵ The body starts processing the simple or compound sugar as soon as it enters your mouth.²

Fun Fact:

Products with fast-acting carbohydrates, such as honey, immediately taste sweet. The sugar occurs in its simplest form, either as glucose or fructose.

Foods that contain slow-acting carbohydrates, such as whole-grain bread, only taste sweet after prolonged chewing.² Enzymes from the saliva start splitting up long-chain carbohydrates into polysaccharides or monosaccharides in the mouth.

The digestive tract continues this splitting in the small intestine, producing simple, fast-acting carbohydrates that enter the bloodstream.² Once there, blood glucose levels increase, and insulin is secreted. The hormone is then responsible for transporting sugar into the cells, after which the blood glucose level falls again.

The body’s cells, particularly those in internal organs, muscles, and the brain, need sugar to keep functioning: exercise, thinking, and metabolic processes depend on glucose.² If too much energy is available from carbohydrates for the moment, the body can store the excess as glycogen in the muscles or as body fat.^2,4

But what exactly are fast-acting carbohydrates, and which foods contain slow carbs?

What are fast-acting (short-chain) carbohydrates?

Monosaccharides, such as glucose, fructose, and galactose, are also referred to as short-chain carbohydrates.⁴ They consist of just one sugar molecule, meaning they can be processed directly by the body without going through a splitting process.

Products that contain these sorts of fast-acting carbohydrates are used for a rapid burst of energy and are very good, for example, for the first-aid treatment of non-severe hypoglycemia. These are some products that contain fast-acting, short-chain carbohydrates:⁶

• Food made from wheat flour, such as baguettes or pretzels
• Cornflakes
• Ripe bananas and dried fruit
• Ice cream, gummies, cookies, malt sugar, grape sugar, and waffles
• Fruit juices
• Lemonade
• Honey and jam

Convenience food and dairy products also frequently contain added monosaccharides. It’s important to know that fast-acting, short-chain carbohydrates must be stated on the label,⁷ but will not necessarily be clearly shown as “sugars” in the list of ingredients so you need to look carefully at the details on the package or go straight for a natural product.

Disaccharides are composed of two sugar molecules.⁴ This group includes table sugar (sucrose) and milk sugar (lactose). Disaccharides are also short-chain carbohydrates and are fast-acting.

Medium-acting carbohydrates

The chains of medium-acting carbohydrates are a little longer, so they break down more slowly.⁴ They cause blood glucose levels to rise more slowly than fast-acting ones. Medium-acting carbohydrates are found in:⁶

• Wheat and rye bread, pita bread, and whole-grain crispbread
• White long-grain rice, white round-grain rice, fast-cooking rice, and basmati rice
• Oat flakes and semolina
• Pineapples, mangos, plums, and mandarin oranges
• Honeydew melon and cherries
• Mashed potatoes (homemade) and boiled carrots

What are slow-acting (long-chain) carbohydrates?

Slow-acting carbohydrates are a complex type of carbohydrate made up of long sugar molecule chains.⁴ That’s why they are also called long-chain carbohydrates. It takes the digestive system a considerable amount of time to break down and transport them to the bloodstream, resulting in a slow effect on blood glucose levels. This effect is particularly pronounced with whole-grain products where the carbohydrates are “packed” inside the fiber, which further delays their breakdown into glucose.

These are typical foods that contain slow-acting, long-chain carbohydrates⁶:

• Whole-grain bread and whole-grain pasta
• Bulgur wheat, brown rice, wild rice, and buckwheat
• Apples, pears, berries, grapefruit, oranges, and peaches
• Legumes, such as beans, lentils, and peas
• Sweet potatoes
• Nuts and seeds, such as hazelnuts, walnuts, and almonds
• Salads (without dressing)

These foods contain slow-acting carbohydrates and therefore have only a moderate effect on blood glucose levels.

The glycemic index (GI) provides information on how quickly certain foods cause blood glucose levels to rise.⁸ GI can be used as a guide for nutrition. Foods with fast-acting carbohydrates, made of monosaccharides and disaccharides, have a higher glycemic index and are considered to be relatively unhealthy. In most cases, they cause blood glucose levels to rise quickly and leave you feeling full for only a short time. 

By contrast, healthy, high-quality foods have a low glycemic index because they contain mostly slow-acting carbohydrates.⁸ The body takes a long time to break down slow-acting carbohydrates. As a result, they leave you feeling full longer and have a slower effect on blood glucose levels.

For people with diabetes, slow-acting carbohydrates with a low glycemic index should be favored while also ensuring that their nutritional content meets the recommended dietary guidelines for fiber, sugars, saturated fats, and protein.⁹

For people with type 2 diabetes, the gentle effect on blood glucose and insulin levels of low GI diets has been shown to contribute to an improvement in health risks.¹⁰

Everything you need to know about the glycemic index.

Which carbohydrates are suitable at different times?

Foods that contain slow-acting carbohydrates are an important part of our daily diet.^4,9 They help to make you feel full and help your digestive system function. These products also often contain many valuable micronutrients and dietary fiber so you can consume them with every meal.

Compared to long-chain carbohydrates, you must be more careful when it comes to short-chain carbohydrates.⁴ Fast-acting carbohydrates, such as monosaccharide glucose, have an immediate effect on blood glucose, which is important for people with diabetes. 

Foods with fast-acting carbohydrates are useful when the body needs energy immediately. However, you should generally consume them in smaller amounts than slow-acting carbohydrates.

Both fast-acting and slow-acting carbohydrates have a place in a balanced diet. Think carefully about how to match consumption with insulin therapy if you need it; your diabetes team can help.

What time of day must I stop eating carbohydrates?

If you have a high tolerance, you can consume fast-acting and especially slow-acting carbohydrates at any time of the day. But monitor how your body responds by measuring your blood glucose levels. For example, consider your sleep quality. It’s very important that your diabetes therapy matches your eating habits.

As a person with diabetes, do I have to give up sugar entirely?

No, you don’t have to give up sugar completely. If you match your consumption with your insulin administration and monitor your blood glucose levels, moderate consumption of sweet foods that contain short-chain carbohydrates won’t be a problem.⁹

Processed food is labeled “no added sugar.” Does that mean it contains no sugar at all?

These products contain no added fast-acting carbohydrates, but the food may naturally contain sugar, so it’s not necessarily sugar-free.⁷

Sources:

1. Get to Know Carbs [Internet]. ADA. 1995-2025 [cited 2025 Jul 9]. Available from: https://diabetes.org/food-nutrition/understanding-carbs/get-to-know-carbs
2. Morris AL, Mohiuddin SS. Biochemistry, Nutrients. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. 2024 [cited 2024 Jul 8]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK554545/
3. Wie gesund sind Kohlenhydrate? [Internet]. Stiftung Gesundheitswissen. [cited 2025 Jul 9]. Available from: https://stiftung-gesundheitswissen.de/gesundes-leben/ernaehrung-lebensweise/wie-gesund-sind-kohlenhydrate
4. Campos V, Tappy L, Bally L, Sievenpiper JL, Lê KA. Importance of Carbohydrate Quality: What Does It Mean and How to Measure It? J Nutr. 2022 May;152(5):1200–6.
5. Guidelines Review Committee, Nutrition and Food Safety (NFS). Carbohydrate intake for adults and children: WHO guideline [Internet]. World Health Organization, editor. Geneva: World Health Organization. 2023 [cited 2025 Jul 9]. Available from: https://www.who.int/publications/i/item/9789240073593
6. Holesh JE, Aslam S, Martin A. Physiology, Carbohydrates. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 [cited 2025 Jul 9]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK459280/
7. Nutrition C for FS and A. Added Sugars on the Nutrition Facts Label [Internet]. FDA. 2024 Mar 22 [cited 2025 Jul 9]; Available from: https://www.fda.gov/food/nutrition-facts-label/added-sugars-nutrition-facts-label
8. Kirpitch AR, Maryniuk MD. The 3 R’s of Glycemic Index: Recommendations, Research, and the Real World. Clin Diabetes. 2011 Oct 1;29(4):155–9.
9. The Diabetes and Nutrition Study Group (DNSG) of the European Association for the Study of Diabetes (EASD), Aas AM, Axelsen M, Churuangsuk C, Hermansen K, Kendall CWC, et al. Evidence-based European recommendations for the dietary management of diabetes. Diabetologia. 2023 Jun;66(6):965–85.
10. Skurk T, Bosy-Westphal A, Grünerbel A, Kabisch S, Keuthage W, Kronsbein P, et al. Dietary recommendations for persons with type 2 diabetes mellitus. Exp Clin Endocrinol Diabetes. 2022 Sep;130(S 01):S151–84.